Add a (minimal) test for combined device and host allocation support. Verified combined behavior is happening with Metal by looking at debug output. (Test verifies nothing leaks or crashes.)
Add a (minimal) test for combined device and host allocation support. Verified
combined behavior is happening with Metal by looking at debug output. (Test
verifies nothing leaks or crashes.)

Adjusted error messages to be easier to disambiguate if test fails.


Former-commit-id: 9eef0a5313a43b10ba25b6a406dd95c16609fe62
darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide